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The effect of in-situ reservoir conditions (high temperature and pressure) on the fracture characteristics (aperture, asperities) and behaviour of fluid through the fractures is a very niche study area for varied energy fields like (i) enhanced geothermal energy systems (ii) nuclear energy storage (iii) CO2 sequestration, (iv) petroleum engineering and (v) storage of liquified natural gas. Keeping this in view many researchers have conducted studies to understand the heat and mass transfer processes occurring in the fractures. Along the same lines Tan et al., 2023 published the paper Thermal Effects Promotes Non-Darcian Flow in Heated Rock Fractures to understand the effect of fluid flow through rock fractures at high temperatures. However, the data presented in the paper by Tan et al., Rock Mech Rock Eng 2023 related to (i) the fracture closure, (ii) the time required for the changes along the fracture, (iii) the thermal expansion effect, and (iv) changes to the phase of water while heating requires further explanation and will otherwise result in the dissemination of incomplete information if not taken care of. The same has been further elaborated in the discussion that follows. [ABSTRACT FROM AUTHOR]

We admit, however, that fracture mechanical principles and rock physics elements behind rock fracturing and fault activation may be similar, and that the joint discussion of fracturing and faulting, therefore, has a long tradition in the rock mechanics and rock engineering community. 1 Participants of the '13th EURO-Conference on Rock Physics and Geomechanics - The Guéguen Conference' during the social event at the Winzerberg Potsdam This special issue consists of 19 scientific papers which are based on these conference contributions and all of which address different important aspects relevant to the variety of industrial applications outlined above. This special issue is dedicated to our dear colleague and friend Prof. Ove Stephansson, who was not only one of Sweden's most influential geologists, but also an enthusiastic supporter of the 13th EURO-Conference on Rock Physics and Geomechanics. Special Issue on Rock Fracturing and Fault Activation: Experiments and Models: Selected Papers Presented at the 13th EURO-Conference on Rock Physics and Geomechanics - The Guéguen Conference Held on 2-6 September 2019 in Potsdam, Germany. [Extracted from the article]

The recent rise of methods using Artificial Intelligence (AI) and Machine Learning (ML) has led to similar problems. Although the title was relatively narrow, the editorial addressed the general issue of papers that concentrated on the numerical method development without much relation to rock mechanics or rock engineering. [Extracted from the article]

A response from the author of the article "An Empirical Failure Criterion for Intact Rocks" in the 2013 issue is presented.

This editorial discusses the journal's policy on publishing papers related to coal mining. While papers that address rock mechanics problems using coal as a material of interest may be considered, the journal recognizes that some studies focused solely on coal mining may not have broader significance for understanding other rocks or rock engineering scenarios. Therefore, studies specifically focused on coal mining without broader significance, including studies on coal mining technology and coal mechanics in the context of specific coal mining methods, will not be considered within the scope of the journal. The publisher remains neutral on jurisdictional claims and institutional affiliations. [Extracted from the article]

A response by G. Anagnostou and L. Canteini to a discussion on their article "On a Paradox of Elasto-Plastic Tunnel Analysis," is presented. The article focused on the counter-intuitive feature of behaviors exhibited under certain conditions. The authors explain the nature of simplifying assumption of zero gravity and use a simplified model to show that the elasto-plastic tunnel paradox persists in the presence of body forces.

Special thanks are due to the 2020 ARMA Symposium technical committee and the Rock Mechanics and Rock Engineering editorial team. Invited authors expanded their original conference papers prior to submission to Rock Mechanics and Rock Engineering. This special issue contains papers based on contributions submitted to the 54th US Rock Mechanics/Geomechanics Symposium. [Extracted from the article]

The article discusses the issue related to evaluation of basic friction angle ɸ in context to the saw-cut rock surfaces. It mentions S.R. Hencher, scientist in context to the contribution of useful references related to the basic friction angle with low values, and highlights the work of Hencher regarding the laboratory estimatation of the angle for the input of the formulation of peak shear strength criterion.

Based on the DP (Drucker–Prager) yield criterion, a new semi-analytical method for hydraulic–mechanical coupling in circular tunnels is proposed which takes into account the evolution of the permeability coefficient by incorporating it into the seepage equation. The definite condition is supplemented through the method of equal flow at the elastic–plastic junction. Constitutive equation and yield criterion are expressed by effective stress. When the pore water pressure drops to zero, the solution presented in this paper can degenerate into the classical Lamé solution in elastic region, and the solution obtained after yielding is consistent with the Drucker–Prage's solution. The numerical simulation method is used to verify the proposed solution, and then, the sensitivity of the strength parameters is discussed. The results show that the radius of the plastic zone decreases with increasing cohesion C and internal friction angle φ°, and the decreasing tendency is more sensitive at lower cohesion and internal friction angle. Higher initial yield stresses are easily obtained by increasing the values of these two parameters appropriately. The solution presented in this paper is not applicable when the internal friction angle is greater than 40°. Considering the intermediate principal stress, the solution presented in this paper shows a higher initial yield in-situ stress than that of MC (Mohr–Coulomb) and Hoek–Brown (HB) materials. Compared with the solution of DP criterion, the solution in the current work has a larger range of plastic region, and is more obvious in high in-situ stress area. In addition, the initial yield stress also increases linearly with the head loss. Highlights: A new closed semi-analytical solution of hydraulic–mechanical coupling in circular tunnel is proposed using equivalent permeability coefficient. The hydraulic–mechanical coupling equation takes into account the action of intermediate principal stresses. The solution proposed in this paper can degenerate into the classical Lamé solution when pore water pressure is equal to zero. [ABSTRACT FROM AUTHOR]

Block collapse is one of the most common geological disasters that occurs during the construction of tunnels in blocky rock masses and is characterized by sudden events and severe damage, which seriously threatens the safety of tunnel construction. To achieve effective control of block collapse, model experiments on tunnel block collapses with a scale of 200 × 125 × 23 cm were conducted to study the effect of the tunnel span on disasters in this paper. The height of block collapse increases with increasing tunnel span, which verifies the accuracy of the discontinuous deformation analysis method (DDA) in simulating block collapse by comparing the experimental results with the DDA simulation results. Then, the influencing factors of block collapse disasters were selected, and the effects of different influencing factors on tunnel block collapse were simulated based on the DDA method. According to the simulation results, the height and area of block collapse were obtained, and the influence laws of each factor on the height and area of block collapse were analyzed. Based on that, the impact levels of each factor on block collapses were divided. Finally, based on the K-means method, tunnel block collapses were divided into five levels according to the height and area of the block collapse, accounting for 28.6%, 32.5%, 23.4%, 11.7%, and 3.9%, respectively. To avoid block collapse, disposal measures were provided for different levels of tunnel block collapse in this paper, which can provide theoretical guidance for the safe construction of tunnels in blocky rock masses. Highlights: Large-scale model experiments were conducted to study the effect of the tunnel span on block collapse. Factors influencing block collapse in tunnels were analyzed based on the DDA simulation results. Block collapses were divided into five levels according to the severity of the disaster based on the K-means method. Standards for prevention measures were established according to the different levels of tunnel block collapse. [ABSTRACT FROM AUTHOR]

As mining activities are expected to develop at greater depths, seismic responses to the blasting of development drift segments are expected to increase and present a greater hazard. A database of 379 development blasts was created for a mining site, recording seismic responses related to these blasts and rock mass structural and geologic properties associated with the drift segment. A random forest, multivariate statistical predictive model was developed with 75% of the drift segments. The model's performance was validated by analyzing 100 drift segments that were not used to create the model. The improved understanding of the variation in the intensity of seismic responses to development blasting through the sum of the seismic moment of the events is a clear benefit of random forest model development for the case study. In addition, the development of the predictive random forest model provides a tool for decision-makers to select performance criteria thresholds that they deem acceptable. The threshold selected would depend on the risk appetite of the decision-makers. The proposed approach provides quantitative data on the distribution of seismic hazards associated with development blasting which managers can rely on. Combining the proposed approach with current seismic protocols used at different mine sites could improve our management of seismic risk associated with development blasting. Using the predictive model for the sector and period studied has shown a potential to increase the accuracy, sensitivity, and precision for anticipating a high-intensity seismic response to a development blast. Highlights: This paper demonstrates the potential benefits of considering geologic and structural properties of the rock mass for managing seismic hazards associated with development blasting. The paper shows how developing predictive random forest models could be used to understand, manage, and communicate the seismic hazard related to blasting of development drift. The random forest models developed with the training data were used to establish thresholds for different risk levels. These thresholds were then applied to the test data to evaluate the actual performance of the models in terms of accuracy, sensitivity, precision, and F1-score at different risk appetite levels. Using the predictive model for the studied period could have increased the accuracy, sensitivity, and precision for anticipating a high-intensity seismic response to a development blast (log (ΣM0)). [ABSTRACT FROM AUTHOR]

High-voltage electric pulse rock-breaking technology has the potential to improve drilling speed significantly. Numerous mechanism studies and laboratory experiments have laid the foundation for developing high-voltage electric pulse rock-breaking technology. However, the mechanism of medium ion channels growing in three dimensions has yet to be fully understood. Based on this, the paper constructs a prediction model of the electrical breakdown trajectory in three dimensions using the finite difference method. The model considers the dielectric properties of the mineral particles inside the rock. The paper then simulates and analyzes the generation of plasma channels caused by the electrical breakdown of rock in space. This is done using the MATLAB numerical simulation software, in combination with the analysis of the electric rock-breaking circuits and the Probabilistic Development Model (PDM). The laboratory experiment on electric rock breaking verifies the prediction model for the trajectory of electrical breakdown. The research results show that the plasma channel generated by electric breakdown in rocks is a spatial curve that expands between the high-voltage electrode and the ground electrode. The breakdown points generated near the high-voltage electrode are more concentrated than those near the ground electrode, forming the primary fragmentation zone. The area near the ground electrode is the secondary fragmentation zone. The voltage difference between the high-voltage level and the grounding electrode significantly influences the crushing effect. A higher voltage leads to a more pronounced crushing effect at the high-voltage electrode. The research results have definite guiding significance for the engineering application of high-voltage pulse rock-breaking technology and the design of downhole drilling tools. Highlights: In this paper, a three-dimensional trajectory prediction model of electrical breakdown is established to study the electrical breakage of rock. The electric breakdown three-dimensional trajectory prediction model is utilized to simulate electrical breakdown in rock under various voltage amplitudes. The reliability of the three-dimensional trajectory prediction model is indirectly verified by the laboratory experiment with an electric pulse rock breaking. [ABSTRACT FROM AUTHOR]

The buried diagenesis and dolomitization in the deep stratum of coral reef island enable the Coral reef limestone (CRL) to exhibit a significant distinction compared with the calcified coral skeleton in mechanical properties. This paper utilized the Split Hopkinson Pressure Bar and X-ray computed tomography to investigatethe dynamic mechanical characteristics of deep CRL, revealing the effect of free water on stress–strain behaviors and the energy evolution law of CRL under high-rate impact. Furthermore, the concept of pore tensor proposed in our previous investigation was introduced to modify the dynamic constitutive model. The results indicate that the dynamic compressive strength (DCS) of CRL specimens is strongly related to stress rate, whereas the dynamic elastic modulus and DCS are independent of strain rate. The peak strains of dry specimens are significantly greater than those of saturated ones at comparative loading rates, and the viscosity effect of free water at a high loading rate impedes the deformation of CRL specimens. Moreover, the energy absorption efficiency of CRL is significantly higher than that of Bohus granite and Solnhofen limestone. Dry specimens have higher energy absorption efficiency under the same energy input, but the dynamic increase factor (DIF) of saturated specimens is larger under the same energy absorption efficiency. For the determination of DIF, this paper adopted the Back Propagation Neural Network model trained by pore tensor to predict the quasi-static compressive strength of CRL specimens, which solved the problem of considerable discreteness in mechanical parameters of CRL. The critical transition value of input energy proportion that the DIF increases rapidly is about 30%. In addition, the constitutive model modified by pore tensor reflects the influence of pore structure and water content of CRL specimens, and well describes the characteristics of dynamic elastic modulus and peak strain of deep CRL independent of loading rate. Highlights: Dynamic compressive strength of coral reef limestone strongly correlates with stress rate and weakly correlates with strain rate. The machine learning-based method for predicting quasi-static strength improves the determination accuracy of dynamic increase factor of coral reef limestone. The critical transition value of input energy proportion that the dynamic increase factor increases rapidly is about 30%. The modified constitutive model reflects the effects of pore structure and water content of coral framework limestone. [ABSTRACT FROM AUTHOR]

During the underground excavation, various flaws are frequently observed, which significantly decrease the mechanical properties of surrounding rock, as has been proven. As known, grouting technology is a means of reinforcement that efficiently enhances the stability of underground engineering. However, existing studies have mainly focused on increasing the strength of the slurry stone, and relatively less attention has been paid to the poor bonding effect at the interface between the slurry and the rock mass. In this work, we proposed to adopt slurry containing nano-silica sol to reinforce the fractured rock mass. Based on the mechanical model of micro–nano-siliceous slurry reinforcement for single fractured rock masses established in this paper, the effect and the mechanism of micro–nano-siliceous slurry reinforcement for fractured rock were explored through theoretical analysis, laboratory tests, and numerical simulations. Results showed that the application of micro–nano-siliceous slurry increased the friction and the bonding force at the slurry–rock interface, which led to a (σn·Δμ + Δc) reduction of the effective stress. The optimum incorporation rate of the nano-silica sol was determined to be 3% based on these indicators including slurry fluidity, slurry stone rate, and strength and deformation parameters of slurry stone. The peak strength of the sandstone specimens reinforced by the micro–nano-siliceous slurry increased by 16.49% those of the sandstones reinforced by the ordinary slurry, and the elastic energy accumulated by the former was higher. Scanning electron microscope images revealed that the incorporation of nano-silica sol distinctly improved the pore characteristics of the slurry-rock interface. Finally, on the basis of 1790 ventilation roadway of Shiyakou coal mine, the effectiveness and the feasibility of micro–nano-siliceous slurry for reinforcing underground engineering surrounding rocks were verified using numerical simulation. Highlights: This paper developed a new micro–nano-siliceous grouting material. A mechanical model for fractured rock mass reinforced by new slurry was proposed. The optimal incorporation rate of nano-silica sol was determined by various means. This study demonstrated the effectiveness of the new grouting material in reinforcing fractured rock mass by control lab experiments and numerical simulation. The mechanisms of micro–nano-siliceous slurry to improve slurry–rock interface were revealed. [ABSTRACT FROM AUTHOR]

In the construction of tunnels in the southwestern region of China, the super-deep burial depth results in extremely high ground stress. In the event of encountering soft rock, there is a potential risk of triggering severe deformation disasters, leading to issues such as support structure damage, project timeline delays, and a significant increase in costs. Therefore, there is an urgent need to propose an advanced stress-release technology to address the high-ground-stress issue caused by the super-deep burial in tunnels. The paper takes the Haba Snow Mountain Tunnel as an example and employs various methods such as numerical simulation and on-site monitoring to investigate the advanced stress-release technology of an advanced center drift in super-deep-buried soft-rock tunnels. It also determines the reasonable parameters for the advanced center drift. The research results indicate that setting the area of the advanced center drift to 60% of the area of the main tunnel is more reasonable. Additionally, positioning the vault of the advanced center drift at a distance of 2.0 m from the vault of the main tunnel can reduce stress concentration at the main tunnel's vault. It is also recommended that the main tunnel excavation should lag by two times the tunnel diameter after the advanced center drift excavation, contributing to the effective control of deformation in the main tunnel. The application of the aforementioned design parameters of the advanced center drift in the Haba Snow Mountain Tunnel resulted in a reduction of approximately 29.65% in the deformation of the main tunnel, ensuring the safety of the support structure. The research outcomes presented in this paper can provide valuable reference for similar engineering projects. Highlights: Established a numerical model capable of simulating high-stress soft-rock tunnels and verified it. Proposed a technique for releasing ground stress in high-stress soft-rock tunnels based on advanced center drift. In-depth discussion was conducted on the mechanism of releasing ground stress in the main tunnel through the advanced center drift. The technical parameters of the advanced center drift have been successfully applied on site, and the deformation of the tunnel has been reduced. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses papers on rock dynamics and applications on topics including dynamic response of rock masses in deep mines, rock bursting in discontinuous deep rock masses, and numerical testing and dynamic fracture modelling.

A three-dimensional representation of the random distribution of fractures in rock masses, known as the discrete fracture network (DFN), is widely used to analyze the stability of jointed rock slopes. In this paper, a new framework for constructing three-dimensional DFN models in rock masses has been proposed to overcome the limitations of conventional circular or polygon-based models. The framework utilizes the universal elliptical disc (UED) model and integrates it with the discrete element method in 3DEC for the stability evaluation of jointed rock slopes. This paper starts by introducing the basic principles of the UED model. The procedures for constructing a three-dimensional DFN using the UED model is then outlined. In the present study, a case study of a rock slope in Zhejiang Province, China is used to demonstrate the implementation of the proposed framework. A comprehensive comparative study is conducted to investigate the impacts of several UED model parameters, including the ratio of major axis to minor axis and rotation angle, and discontinuity density, on the stability of rock slopes and compared to the conventional Baecher disc model. The results show that the framework can effectively integrate the UED model into 3DEC and provide a realistic representation of the three-dimensional DFN, leading to improved accuracy and efficiency in the stability evaluation of jointed rock slopes. The framework also shed light on the interactive effects of the UED model parameters and discontinuity density on the rock slope stability, providing a strong reference for using the UED model in constructing DFN models for rock slopes. Highlights: Three-dimensional discrete fracture networks (DFNs) of rock masses are successfully constructed using the universal elliptical disc (UED) model. Python subroutines are developed in 3DEC to construct 3D DFNs using the UED model for improved discrete element analyses of jointed rock slopes. The effects of several characteristic parameters of the UED model on the stability of jointed rock slopes are investigated. Based on a key block model, the mechanisms underlying the relationship between jointed rock slope stability and fracture discontinuity are rigorously explained. The advantages of the UED model over the conventional Baecher disc model are comprehensively discussed based on a rock slope case study. [ABSTRACT FROM AUTHOR]

Salt caverns are internationally recognized as excellent facilities for underground energy storage. Creep shrinkage deformation will occur in deep salt caverns under the action of high-ground stress, and it is a key factor to evaluate the safety of salt caverns. However, there has been no salt cavern creep shrinkage mechanism research on ultra-deep salt caverns. In this paper, the creep shrinkage mechanism of an ultra-deep salt cavern is established from the theory, numerical simulation, and field application. First, the actual creep field experiments (pressure testing and sonar testing) of the salt cavern (depth ≥ 2000 m) are carried out. Second, theoretical models of salt cavern creep shrinkage are established from four influence aspects (creep shrinkage, heat conduction, salt dissolution, and brine permeability). Third, a 3D geological model is built to analyze the creep stability of deep salt caverns based on their field conditions. The novelty of this paper is analyzing the creep shrinkage of the ultra-salt cavern by the theoretical model, numerical model, and field experimental data systematically. The results show that the shrinkage of the salt cavern and brine thermal expansion is key factors leading to pressure lifting of the salt cavern, which accounts for 0.6121 and 0.2147 in the four influence aspects. Three creep phases are obtained: rapid rising stage, steady rising stage and decelerating rising stage. The study provides a reference for the creep shrinkage and field application of salt caverns. Highlights: Creep field experiments of the salt cavern are carried out. Theoretical models of ultra-deep salt cavern creep mechanisms are built. An ultra-deep 3D salt cavern creep numerical model is built to analyze the creep stability of salt caverns. A mathematical model is proposed to analyze the factors that influence salt cavern creep. [ABSTRACT FROM AUTHOR]

In this paper, for the engineering problem of tunnel deformation damage caused by a tunnel–landslide orthogonal system landslide in northwest China, based on similar theory, a variety of materials and their ratios were explored to make similar models according to the physical and mechanical parameters of the site engineering rock, indoor physical model tests were constructed, and monitoring techniques, such as speckle displacement strain measurement, strain acquisition, and pressure sensors, were used to test the macroscopic deformation characteristics; through the detailed analysis and law summary of the test macroscopic deformation characteristics, displacement, anchor cable axial force, and strain data, it was finally found that under the orthogonal system of tunnel and sliding surface, the damage form of tunnel lining is mainly compression damage and shear damage, and the cracks are mainly tension cracks, and the effect of tunnel primary lining on the surrounding rock support is extremely limited under the action of huge sliding force; combined with the analysis of the model deformation characteristics and the axial force curve of the anchor cable during loading, it was found that the axial force change curve is consistent with the NPR Newton force curve can be used to evaluate slope stability and predict the occurrence of landslide disasters. Highlights: This paper focuses on the tunnel and surrounding rock damage evolution characteristics under tunnel–landslide orthogonal and tunnel crossing sliding surface conditions. The evolution of displacement field, NPR anchor cable axial force, and strain field were revealed using indoor large similar physical model test. The mechanism of tunnel–landslide interaction within the slope and the deformation and damage characteristics of the tunnel surrounding rock were analyzed to reveal the slope deformation law under different loads. The stress change law of constant-resistance large deformation anchor cable (NPR anchor cable) is explored, and a landslide warning mechanism based on constant-resistance large deformation anchor cable is established. [ABSTRACT FROM AUTHOR]

The shale oil reservoirs of the Lucaogou Formation in the Jimsar Sag developed a large number of natural fractures and thin interlayers. The expansion of hydraulic fractures is full of randomness and unknowingness due to this complex geological feature. Since the multi-cluster fracturing process with small well spacing is widely used, the growth of hydraulic fractures requires a combined consideration of competition between perforation clusters and interference between wells. From the perspective of engineering application, this paper summarizes the fracture propagation law under single-fracture, multi-cluster fracturing, and adjacent well interference from simple to complex based on three-dimensional discrete element methods and a large number of field data. Then the key controlling factors of fracture group morphology were quantitatively analyzed. Comparative analysis reveals that under complex geological conditions, a single crack can still be described by existing theories, but the expansion law of the fracture group is not a simple superposition of single fractures. The competitive expansion of multiple fractures is reflected in the Fracture Surface Area (FSA) differences between clusters, asymmetry, and irregularity. Moreover, under inter-well stress interference, the hydraulic fractures of the posterior-construction well will tend to expand in the opposite direction of the first-construction well. The optimal well spacing of the Jimsar shale oil reservoir is 200–300 m. Highlights: In this paper, there are five highlights worthy of attention: This paper discussed the fracture propagation law under actual multi-cluster fracturing conditions through three aspects: 1. Clarifying the difference between the expansion law of multi-fracture competition and traditional cognition by comparing the morphology of fracture group and single-fracture; 2. Competitive expansion modes were quantitatively analyzed through FSA curves and inter-cluster stress interference. 3. Based on inter-well stress interference, a more complex simulation of actual working conditions was carried out to elaborate further on the influence of inter-well stress interference on fracture expansion. This paper compared the penetration expansion ability of hydraulic fractures under the condition of single fractures and fracture group. We quantitatively analyzed the dynamic characteristics of multi-fracture competitive expansion through Fracture Surface Area (FSA) and evaluated the morphological attributes of fracture groups under inter-cluster and inter-well stress interference. The stress interference range and the increment of the minimum horizontal in situ stress caused by fracturing adjacent well under different well spacings were quantitatively analyzed. The research was based entirely on the actual working conditions of the oil field, and the microseismic data and simulation results were thoroughly compared and demonstrated, which ensured the reliability of the conclusions. [ABSTRACT FROM AUTHOR]

This paper investigates potential differences between creep and consolidation in mechanised tunnelling through squeezing ground, placing focus on the practical question of using experiences gained from existing tunnels about the required thrust force as a reference for tunnels of different diameter or adjacent tunnels. The investigations focus on two aspects. First, the effect of the tunnel diameter on the risk of shield jamming is examined. The paper demonstrates that larger-diameter tunnels are more favourable in poor-quality ground, while the opposite holds in better-quality ground, as well as in the case of pronouncedly time-dependent ground behaviour due to consolidation or creep. Second, the effect of a tunnel on the required thrust force in a neighbouring tunnel built later is examined. The paper shows that this interaction effect is particularly important in water-bearing ground of low permeability, where the drainage action of the first tunnel induces pore pressure relief and ground consolidation in an extensive area, leading to a remarkable reduction of the thrust force in the second tunnel. Conversely, in the case of creep the interaction is negligible even under extremely squeezing conditions, due to the fundamentally different nature of the purely mechanical rheological processes from coupled hydromechanical processes. The presented investigations into the transferability of experiences are valuable for tunnelling practice, in cases of twin tunnels as well as in situations where a smaller-diameter tunnel is constructed prior to the main tunnel (e.g. a pilot tunnel for exploration, advance drainage or ground improvement), or also the opposite (e.g. upgrade of a road tunnel by later construction of a safety tunnel with a smaller diameter). Highlights: The rate of ground deformations is independent of diameter in the case of creep, but extremely increases with decreasing diameter in the case of consolidation. The risk of shield jamming is lower in larger-diameter tunnels crossing poor-quality ground without pronounced time-dependent behaviour. The risk of shield jamming is lower in smaller-diameter tunnels crossing ground of higher quality or exhibiting pronounced time-dependent behaviour. The effect of adjacent tunnel on shield jamming risk is generally minor in the case of creep, even for extreme squeezing conditions and large overcuts. The effect of adjacent tunnel on shield jamming risk is remarkable in the case of consolidation, leading to a substantial reduction of the thrust force. [ABSTRACT FROM AUTHOR]

Burden is one of the most important parameter which plays an immense role in rock breakage using explosive energy. It is very difficult to select the optimum burden under different geotechnical and geomining condition. The selection becomes more critical when the excavation method is underground as very few researches or studies have been carried out in such cases. Therefore, in this paper, numerical simulation-based approach has been employed to estimate the optimum burden for ring blasting. The numerical models were created under different scenario including blasthole diameter and strength of the rock. The RHT concrete constitutive model has been used in this paper to study the damage contour under different circumstances. The model output has been used to calculate the volume of rock damage along the free face. This damage volume is further correlated with various parameters. This correlation shows that the damage volume along the free face initially rises with the increase in burden and then starts decreasing after a certain value. The burden value which recorded the maximum volume of rock excavated along the free face has been considered as the optimum burden in this paper. The trial blast has been also carried out at the experimental site for twenty-four rings. The optimum burden achieved from the experimental trial for 102 mm blasthole diameter and rock having uniaxial compressive strength as 92 MPa lies between 2.4 and 2.65 m. However, the optimum burden predicted through developed empirical equation from the numerical model is 2.6 m. The study shows that the simulation-based prediction exhibits a minor deviation within 15% with the experimental result. Hence, to assess the optimum burden under various geotechnical and geomining circumstances, numerical simulation-based approach may be carried out using the proposed methodology. Highlights: Burden for ring blasting is optimised using different numerical simulation models. Volume of rock excavated along free face (EV) has been calculated in each case. The optimum burden from experimental trial and empirical data is 2.4–2.65 m. The optimum burden predicted from developed model is 2.6 m. [ABSTRACT FROM AUTHOR]

Electric impulse drilling (EID) technology has great industrialization potential since it has unique advantages in crushing hard rocks of deep formation. To study the mechanism of dielectric breakdown is of great significance for accelerating the practical application of EID technology. In this paper, the dielectric breakdown damage model (DBDM) is established based on the heterogeneous granite model. On the basis of DBDM, the whole process of absolute dielectric breakdown (ADB) is studied, including the formation of the plasma channel and the variation of the electric field. Then, the mechanism of regional dielectric breakdown (RDB) development to ADB is studied, and the influence of peak voltage, electrode spacing and the parameters of a high-voltage electric pulse (HVEP) is discussed. Besides, the efficiency of two dielectric breakdown modes is evaluated. It is found that dielectric breakdown will only occur in the domain where the strength of the local electric field formed by HVEP exceeds the critical breakdown field strength of rock. RDB will occur when the pulse duration is too short or the peak voltage is too low. It is worth noting that RDB can be developed into ADB under multi-HVEPs. Moreover, under the same voltage, there is almost no difference between the failure volume caused by RDB under multi-HVEPs and ADB under a single HVEP. However, the energy consumption of the RDB under multi-HVEPs decreases exponentially with increasing voltage and is always higher than that of the ADB under a single HVEP. When designing the supporting tools and bits of EID technology, the life of insulation, the characteristics of formation, and energy utilization should be fully considered. The related research in this paper aims to provide reference and guidance for the application of EID technology in drilling engineering. Highlights: The variation of the electric field, the change of minerals and the generation of plasma channel in granite during dielectric breakdown is studied. The effect of the characteristics of high-voltage electric pulse (HVEP) on the dielectric breakdown in granite is studied. The mechanism and rock-breaking efficiency of absolute dielectric breakdown (ADB) and regional dielectric breakdown (RDB) are explored. [ABSTRACT FROM AUTHOR]

Abandoned underground mines appear to be very sensitive to the variations in their environment. Due to seasonal environmental changes often related to climate and resulting in changes in the water table and hygrometry, the pillars are often subjected to cyclic variations of saturation. This could affect the short- and long-term stability of these abandoned structures. In this paper, a preliminary study on the behaviour of Château-Landon chalk has been carried out. Samples were extracted from pillars of the abandoned Royer chalk mine (Château-Landon, France). The mechanical behaviour of this chalk was then investigated by performing conventional hydrostatic, triaxial compression, and creep tests under drained conditions at different water saturation degrees and confining pressures. The obtained results show up fundamental aspects of this chalk behaviour. Its high sensitivity to water and confining pressure are described, and a water saturation-induced plastic deformation is analysed. The results presented in this paper provide some key mechanisms that can be used in the framework of back analysis and expertise in underground chalk mines, as well as the data required for the development of a specific constitutive model. Highlights: Compression tests were performed on the Château-Landon chalk at different water saturation degrees (10%, 40%, 60%, 90%, 100%) and confining pressures (0.5, 1, 1.5, 2, 3 Mpa). An analysis of the creep behaviour of this material at a partial and total saturated state was shown. The variation of mechanical properties with water saturation degree and confining pressure was analysed. [ABSTRACT FROM AUTHOR]

Coal pillar is a crucial element formed during underground coal mining, playing a significant role in ensuring coal mining operations are conducted safely. Historically, research on coal pillars has primarily focused on regular-shaped pillars, with limited attention given to special-shaped pillars. Therefore, this paper aims to investigate the special-shaped coal pillar formed after the excavation of the newly constructed 3307 headgate in Dayang Coal Mine, China. The primary object of this paper is to elucidate the mechanism behind the instability failure of special-shaped coal pillars and verify the influence of various geological conditions on their stability. To accomplish these goals, numerical simulation techniques were employed to analyze the stress distribution patterns and plastic zone failure characteristics of special-shaped coal pillar. The investigation revealed the presence of stress concentration phenomena within special-shaped coal pillar, where the stress peak values were directly correlated with the width of the pillar. Moreover, it was observed that as the width of coal pillar decreased, the stress curve gradually transitioned from a bimodal shape to a unimodal shape. Additionally, the ratio of the plastic zone within coal pillar increased as the width decreased, with pronounced tensile failures occurring at sharp corners. Building upon theoretical analysis and numerical simulation results, the interior of special-shaped coal pillar was classified into four distinct zones: damaged zone, progressive zone, stable zone, and special zone. Based on this zoning approach, a stability control technology was developed, employing a combination of "pier pillar support + flexible formwork concrete + grouting reinforcement + high-strength cable and bolt + U-steel." Subsequently, an industrial experiment was conducted to validate the efficacy of this control technology. Field monitoring results demonstrated that the newly excavated roadway exhibited minimal deformation and damage, thereby satisfying the requirements for safe and efficient mine production over the anticipated service life. Highlights: The failure characteristics and bearing mechanism of special-shaped coal pillars were analyzed, and a mechanical model was established, allowing exploration of the influence of different geological conditions on pillar stability. The stress evolution and plastic zone development characteristics of special-shaped coal pillar during the whole cycle were studied. A zoning criterion for special-shaped coal pillar was proposed, along with corresponding control measures, which were successfully applied to the site, effectively controlled the deformation of roadway. [ABSTRACT FROM AUTHOR]

Although squeezing ground may undergo rapid convergences following tunnel excavation, its behaviour is often markedly time dependent due to creep or consolidation. The effects of creep and consolidation on shield tunnelling are comparatively evaluated in two companion papers, with the aim of demonstrating their qualitative similarities and distinctive features. The present, first paper investigates the basic time effects, placing focus on the time development of ground deformations and the complex interaction between ground, tunnel boring machine (TBM) and tunnel support during excavation and during construction standstills. The presented numerical simulations indicate several qualitative similarities between the two mechanisms of time dependency, in respect of the time development of ground deformations, the counter-intuitive behaviour of increasing shield loading with increasing rate of advance under certain conditions, as well as the thoroughly adverse effect of the additional time-dependent deformations taking place during construction standstills on the shield loading. However, they also underscore two prominent differences resulting from the fundamentally different nature of creep (a purely mechanical rheological process) and consolidation (a coupled hydromechanical process): first, the consistently more extensive plastic yielding in consolidating ground, which is partially associated with the seepage forces exerted by the pore water on the solid rock constituents. Second, the role of seepage forces as a potential destabilising agent, particularly for the tunnel face, which does not happen in the case of creep and may be critical for shield and cutterhead jamming. Building upon these investigations, the companion paper compares creep and consolidation with respect to the transferability of experiences about the required thrust force to tunnels of different diameter or to adjacent tunnels. Highlights: Analysis of similarities and differences between the effects of creep and consolidation on mechanised shield tunnelling through squeezing ground. Similar time development of rock deformations and shield loading during advance and during standstills in creep and consolidation. Similar counter-intuitive behaviour of increasing shield loading with increasing advance rate under certain conditions in creep and consolidation. More extensive ground plastification in consolidation than in creep, partially due to the effect of seepage forces on the solid rock constituents. Potential instability due to seepage forces may induce excessive convergences or face extrusion and render shield or cutterhead jamming critical. [ABSTRACT FROM AUTHOR]

Fluid leakage caused by debonding at the cement interfaces driven by low-temperature fluids is recognized as a significant challenge to wellbore integrity. Previous studies have primarily focused on casing/cement interface debonding, without thoroughly analyzing the simultaneous propagation of debonding fractures at the casing/cement interface and cement/formation interface. Furthermore, prior investigations have been conducted based on hydro-mechanical coupling analysis, neglecting the influence of injection fluid temperature on interface debonding. Therefore, this paper aims to reveal the debonding mechanism of the two cement interfaces using a coupled thermal–hydrological–mechanical (THM) modeling approach. Considering the effect of injected fluid temperature, a 3D numerical model of casing-cement-formation is developed for the simulation of the fluid-driven debonding based on the coupled pore pressure cohesive zone method. The influences of various factors including injection fluid temperature, flow rate, viscosity, casing pressure and cement Young's modulus on the cement sheath debonding are studied, and grey relational analysis is carried out on the influence degree of different factors. The results indicate that the fracture propagation pressure (FPP) is higher, and the fracture height is lower at the casing/cement interface compared to the cement/formation interface. When the fluid temperature is considered by the model, the FPP at the cement sheath interface is lower, and fracture initiation and propagation are more likely to occur. The results also demonstrate that higher fluid temperature, flow rate, viscosity and casing pressure increase the FPP and reduce the risk of cement sheath interface debonding. Casing pressure and fluid temperature exhibit a more prominent influence on FPP. The method can effectively predict the propagation of debonding failures at the cement sheath interface, and provide a foundation for the optimization of injection parameters. Highlights: This paper investigates the simultaneous propagation of the fluid-driven debonding of two cement interfaces considering the injected fluid temperature. A novel numerical model considering the fully coupled thermal–hydrological–mechanical processes is developed. Effects of injection fluid temperature, flow rate, viscosity, casing pressure and cement Young's modulus on the interface debonding are examined. A grey relational analysis is conducted to rank the influence degree of different factors. [ABSTRACT FROM AUTHOR]

In the geological disposal of high-level radioactive waste in argillaceous rocks, studying the barrier integrity after gas transport and the pathway closure thanks to self-sealing capacity is a crucial aspect for the safety assessment. This paper presents experimental research in Boom Clay (a potential host rock in Belgium) to evaluate the effectiveness of self-sealing and possible fissure reactivation during a second gas invasion event. Initial water permeability under oedometer conditions was first measured on samples at two bedding orientations, being higher the sample with bedding planes parallel to flow, highlighting marked anisotropy. Then, gas injection tests at a constant volume rate were performed. Results indicated that Boom Clay underwent expansion and degradation during gas injection due to the development of fissures that were quantified using microstructural techniques. The computed effective gas permeability was not significantly dependent on bedding orientation and was slightly larger than the initial intrinsic water permeability. The re-saturation of the samples led to a recovery of the initial water permeability for both orientations, replicating the original anisotropy. The microstructural analyses confirmed the gas pathways' closure, indicating good self-sealing and the regaining of the hydraulic barrier function. However, a small volume of large unconnected pores was detected on undrained unloading before the microstructural study. An additional gas injection after the self-sealing resulted in a higher effective gas permeability and a larger increase in pore volume, suggesting the reopening of fissures generated during the first injection. Finally, the experimental data were compiled within a multi-scale phenomenological model to relate the microstructural information to macroscopic flow transport properties capturing the intrinsic permeability increase on gas invasion and its recovery during self-sealing. Highlights: Gas transport on Boom Clay occurs along fissures, increasing its permeability The re-saturation process induces the recovery of the initial water permeability, demonstrating an excellent self-sealing capacity. A subsequent gas injection after the re-saturation suggests the reopening of previous gas paths. A multi-scale model is proposed to account for the permeability changes derived from a microstructural damage variable. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of the swelling behaviour of a compacted bentonite–sand mixture subjected to hydration under constant volume conditions. Contrary to previous studies, the tested sample was isotropically compacted before being hydrated under constant volume conditions until full saturation was reached. The total axial pressure, total radial pressures at four different heights of the sample, and injected water volume were recorded over time. The experimental data reveal a complex and non-uniform evolution of the axial and radial stresses over time, as well as anisotropy of the total stresses, which persist at the saturated equilibrated state. To gain further insights, a numerical analysis was performed using an advanced hydromechanical framework for partially saturated porous media, accounting for the evolving microstructure of the material. The complex evolution of the total axial and radial pressures with time is attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The good agreement between the numerical results and the experimental data enables validation of the developed framework. Implications for engineered barriers in deep geological disposal of radioactive waste are discussed. Highlights: The swelling behaviour of an isotropically compacted bentonite-based material under constant-volume conditions is investigated. Hydration of the sample generates stress heterogeneity and anisotropy, which persist at the saturated equilibrated state. Numerical modelling shows that the complex evolution of the total axial and radial pressures can be attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The relationship between the local dry density and the radial stress seems to follow the global dry density-swelling pressure trend determined on small-scale samples. [ABSTRACT FROM AUTHOR]

This paper presents several series of multi-stage permeability tests in laboratory with both steady-state and pulse decay methods on core samples of the low permeable Callovo-Oxfordian claystone from Meuse/Haute-Marne (France). The focus is to explain why water permeability values obtained in laboratory with the steady-state method are lower than those obtained with the pulse decay method and those obtained in situ in the Underground Research Laboratory designed for a feasibility of a deep geological repository of radioactive waste. The focus is also to assess the influence of the percolating water chemistry and the duration of resaturation and steady flow stages on the permeability values, and the impact of these permeability measurements on the mechanical properties (elastic coefficients, peak strength). The water permeability values measured with the pulse decay method are homogeneous and around 10−20 m2. Those obtained with the steady flow method are significantly lower (up to one order of magnitude) when using the synthetic water of ANDRA. This difference can be explained by an additional hydration of the clay minerals during steady-state flow which induces a swelling and then a decrease of the interparticle porosity. These physicochemical mechanisms slow down the water flow during the steady flow tests. When using a chemically balanced water and reducing the test duration, the difference is only about half an order of magnitude. Sample resaturation and steady-state flow induce a significant damage of the material which is mainly due to the opening of the bedding planes of swelling clay minerals. Highlights: Laboratory permeability tests with steady state and pulse decay methods were performed on core samples of Callovo-Oxfordian claystone from Meuse/Haute-Marne (France) Permeability values measured with the steady flow method are significantly lower than those obtained from the pulse decay method when using the synthetic water of ANDRA This difference could be explained by an additional hydration of the clay minerals during steady state flow, which induces swelling and slows down the water flow A more accurate assessment of the permeability with the steady state method was obtained using a chemically balanced water and experiment with shorter run time Initial resaturation and steady state flow induce a significant damage of the material which is mainly due to the opening of the bedding planes of swelling clay minerals [ABSTRACT FROM AUTHOR]

A discussion on the article "On a Paradox of Elasto-Plastic Tunnel Analysis," by L. Cantieni and G. Anagnostou, published in a previous issue, is presented. It cites that the article focuses on the paradox that persists in elasto-plastic analyses of tunnels. It argues that counter-intuitive behavior is a stress relief that occurs in the ground prior of the face.

Bimslopes are slopes underlain by block-in-matrix rocks/soils (bimrocks/bimsoils), which are heterogeneous geomaterials with complex behavior. Abundant uncertainties in the mechanical, spatial, and geological nature of bimslopes make it difficult to estimate stability conditions of bimslopes. In this paper, some factors affecting the stability of bimslopes are investigated using the finite-element method. The main contributory factors include Volumetric Block Proportion (VBP), the inclination of blocks, and the strength of matrix/block interfaces. By performance of sensitivity analysis on these factors, a statistically significant equation was obtained to estimate the safety factor of bimslopes using the multivariate regression method. The influence of block configurations and groundwater levels on the stability of bimslopes was then investigated to more confidently use the multivariate equation. Finally, to check the validity of procedures presented in this paper, a real failed bimslope in the Sungun mine (Iran) was investigated. The acceptable results from the Sungun analysis demonstrated the safety factor of bimslopes can be estimated using the practical solution presented in this paper. Highlights: Development of more than 1300 numerical models to analyze the key factors affecting the stability of block-in-matrix slopes (bimslopes). Investigation of the influence of volumetric block proportion, the inclination of blocks, and strength of matrix/block interface on the safety factor of bimslopes. Consideration of the substantial uncertainties affecting the analysis of bimslopes. Presentation of the results of research in the form of applied schemes and tables. Provision of a practical procedure and equation for estimating the safety factor of bimslopes. [ABSTRACT FROM AUTHOR]

Thermal spallation is the erosion of brittle materials subjected to high heat fluxes, such as a hot fire in a concrete structure. While experimental investigations provide valuable data on the spallation process, they lack generality to be extrapolated to other materials or loading conditions. This paper presents a numerical method for predicting bulk quantities of the spallation process, such as the average recession rate, for crystalline materials. The method is based on direct numerical simulations of the microstructure. The crystal grains and their boundaries are modeled with finite elements and cohesive zones. These elements have anisotropic and temperature-dependent physical properties, a novel level of fidelity and one that accurately captures known crystal phenomena including the α – β quartz transition. These models of the crystalline material and the thermal spallation process are validated against experiment results for Barre granite, and then applied to predict the thermal spallation of Martian basalt. Highlights: This paper presents a high-fidelity model of a process called thermal spallation. Each individual grain within a rock is simulated directly, with material properties that are both temperature- and direction-dependent. This computational model reproduces experimental results for Barre granite. The model is then used to predict thermal spallation of Martian basalt, which is relevant to landing large robot and human missions on Mars. [ABSTRACT FROM AUTHOR]

Rock images play a vital role in providing data for engineering geological studies. However, low-light (a.k.a. dark) rock images are often obtained, and the existing methods for image enhancement, whether they are software-based, algorithm-based, or data-driven-based, are incapable of addressing the issue. This paper establishes a novel unsupervised deep learning (DL) model that caters to low-light rock image enhancement. The light enhancement process is carried out automatically and pixel-wise by utilizing the deep curve estimation (DCE) algorithm and a convolutional neural network (CNN). This study establishes a rock image dataset, encompassing diverse (1) rock types (tuff, siltstone, & granite groups), (2) light levels (2400 lx, 1200 lx, &100 lx), (3) color temperatures (5500 K, 4200 K, & 3000 K), and (4) surface conditions (wet & dry). The novel DL model is developed based on (1) high-order curves of the DCE algorithm, (2) an elaborate CNN architecture with step-wise convolutions and a squeeze and excitation module, (3) three non-reference loss functions, and (4) an alerting level for better generalization. The DL model possesses several advantageous features: (1) automatic light enhancement of rock images without subjective inputs, (2) independence from paired data, which requires manual retouching, (3) pixel-wise adjustment of intensities across color channels, (4) preservation of rock details without distortion by the attention mechanism, (5) high inference speeds, e.g., ~ 3 ms per rock image, and (6) most importantly, state-of-the-art performance when compared to 10 other widely-used enhancement methods, both visually and quantitatively, by three evaluation indices. Despite being designed and trained for rock images, the DL model also demonstrates impressive performance on non-rock images. Highlights: This study establishes a rock image dataset, encompassing diverse rock types, light levels, color temperatures, and surface conditions. This paper establishes a novel unsupervised deep learning (DL) model that caters to low-light rock image enhancement, automatically and pixel-wise. The DL model achieves state-of-the-art performance in enhancing low-light rock images when compared to 10 other widely-used light enhancement methods, both visually and quantitatively. [ABSTRACT FROM AUTHOR]

In-situ stress measurement is an important prerequisite for underground engineering excavation design and surrounding rock stability analysis. However, it is still a difficult problem to complete in-situ stress measurement quickly and accurately, especially in deep vertical borehole with more complex environment. Aiming at the existing problems, a new in-situ stress measurement method is proposed in this study. The principle of in-situ stress calculation based on the borehole diametrical deformation analysis is established, and a borehole diametrical deformation measurement equipment which can realize the directional measurement of multi-directional borehole wall displacement is developed according to this principle. The traditional single action double-tube drilling tool is optimized, and the corresponding in-situ stress measurement process is formulated, forming a rapid in-situ stress measurement method with the cooperation of drilling tools. The measurement method has been applied to the in-situ stress measurement at − 410 m and − 500 m levels in Zhangfushan deposit of Jinshandian Iron Mine, and effective diametrical deformation data of 6 measuring points were obtained. The measurement results show that the optimized single action double-tube drilling tool has the ability of accurate drilling and complete coring, and can assist in the rapid measurement of in-situ stress. The principal stress at the 6 measuring points basically increases with the increase of depth, and the direction of the maximum horizontal principal stress is about SN, which is basically consistent with the existing in-situ stress data. The results show that the method proposed in this paper can realize the rapid and accurate measurement of in-situ stress in vertical borehole. Highlights: A principle for calculating in-situ stress based on the analysis of diametrical deformation characteristics of boreholes has been established. A device has been developed that can measure diametrical displacement of borehole in multiple directions. The method was used for in-situ measurement and the accuracy of the measurement results was verified through historical data. [ABSTRACT FROM AUTHOR]

Flysch formations are widespread all around the Mediterranean basin and in many other parts of the world. Their special fabric, characterized by a strong heterogeneity due to alternations of often deformed lapideous and fine-grained layers and by a network of discontinuities, brings to light the strong limitations of the geo-engineering sciences when dealing with such complex materials. Based on the results of an investigation carried out in a long section of the Southern Apennines, the paper examines this problem focusing in particular on the role played by material fabric at different scales on the hydraulic and mechanical behavior of the deposit. Highlights: The mechanical behaviour of the Red Flysch formation is carefully analysed with a particular attention to the influence of the soil fabric at different scales. The Red Flysch formation is described within the frame of the structurally complex formations The results of a wide in situ and laboratory investigation on a Red Fysch deposit are collected and interpreted The role played by material fabric at different scales on the hydraulic and mechanical behavior of the deposit is evidenced. The strong limitations of the geo-engineering sciences when dealing with such complex materials are highlighted [ABSTRACT FROM AUTHOR]

The existence of confining pressure is an important factor that can't be ignored in rock cutting process. Rocks show high strength and strong plasticity under confining pressure condition (CPC), leading to the low rock-breaking efficiency of drilling bit, which is more prominent in deep formation drilling. However, the understanding of rock-breaking mechanism of Polycrystalline Diamond Compact (PDC) cutter under CPC is still unclear. Given that fact, in this paper, a series of rock cutting experiments on sandstone and granite under different confining pressures and atmospheric pressure conditions were carried out using a self-made true triaxial rock cutting device. The cutting force, mechanical specific energy (MSE) and cuttings mass fraction under confining pressure and atmospheric pressure conditions were studied, and the cutting grooves were analyzed. The results show that the existence of confining pressure greatly increases the difficulty of rock cutting, which is embodied by the increase of cutting force and MSE. In addition, the size of cuttings produced under CPC is smaller, and the proportion of powdery cuttings increases with increasing confining pressure. Meanwhile, the cutting grooves obtained under CPCs are smoother. Microscopically, the existence of confining pressure greatly enhances the contact stress between PDC cutter and rock, resulting in a larger area of dense core when PDC cutter contacts with rock. Confining pressure leads to the difficulty of cuttings discharge to a certain extent, which easily results in the secondary and repeated destruction of cuttings and increases the energy dissipation. In general, confining pressure leads to the lower damage degree of PDC cutter to the rock, which makes the rock difficult to be drilled. The research results can provide some special guidance for rock breaking in deep formation, and provide practical thoughts and enlightenment for rock breaking, bits arrangement and optimization under CPC. Highlights: The cutting experiments under confining pressure condition are carried out by using the self-made rock cutting device that exerts three-dimensional stress on rock, which truly simulates the rock breaking process at the bottom of the well. The rock-breaking performances under atmospheric pressure condition and confining pressure conditions are comprehensively analyzed from the aspects of cuttings mass fraction, cuttings size, cutting groove and coarseness index. The rock-breaking mechanism of PDC cutter under confining pressure condition is investigated and the rock-breaking behavior of PDC cutter under confining pressure condition is studied. Three kinds of changing trends of mechanical specific energy with confining pressure are put forward, and the concept of confining pressure threshold is also proposed. [ABSTRACT FROM AUTHOR]

In this paper, a simplified method for predicting rock mass resistance against tensile forces from rock anchors (anchor) is presented. The interaction of anchors and rock mass was investigated using three-dimensional discontinuous numerical modelling. Several patterns of rock discontinuities were assumed in the numerical modelling while a single anchor is embedded in it. The numerical results show that the existence of a discontinuity set sub-parallel to the anchor significantly improves the rock mass resistance against the tensile force from the anchors. This phenomenon is due to the rock block interlocking at the sub-parallel discontinuity set. Rock block interlocking generates a zone of stress concentration inside the rock mass which has an arch shape (i.e., a pressure arch), resisting against the anchor's tensile force. The load-bearing capacity of the pressure arch plays a significant role in the resistance of the rock mass against the forces from the rock anchor. A voussoir beam analogy was utilised to study the load-bearing capacity of the pressure arch. A simplified analytical approach was developed to assess the load-bearing capacity of the voussoir beams. Then, it was used in combination with the weight of the mobilised rock mass by the anchor to assess the maximum anchoring resistance of the rock mass (anchoring capacity). The suggested method was calibrated by numerical modelling and relevant published pull-out test results. The technique developed in this paper shows the significance of rock block size, shear behaviour of rock discontinuities, Young's modulus of the rock mass, and uniaxial compressive and tensile strength of the intact rock in anchoring capacity of rock masses. Highlights: A theoretical framework is presented on how rock mass and rock anchors interact based on numerical modelling. An analytical method is developed to calculate the anchoring capacity of a rock mass which has at least one joint set sub-parallel to the anchor. The importance of the rock block size and the shear behaviour of rock discontinuities in determining the anchoring capacity of the rock mass is discussed. [ABSTRACT FROM AUTHOR]

For many years, the relationship between permeability and temperature of rocks after high temperature was described by a classical exponential model based on thermos-elastic theory. However, recent experiments have shown that the thermal damage incurred by rocks after high temperatures is irreversible plastic damage. This paper aims to propose a new approach for creating a temperature-dependent permeability model that considers this irreversibility of thermal damage. To achieve this, energy dissipation theory and fractal theory were used to develop a model that describes the irreversible thermal damage evolution and temperature-dependent permeability of rocks after high temperature. The new model was verified through CT and gas permeability experiments. The results indicate that: 1. in the temperature range of 200 ~ 800 ℃, thermal damage develops into an irreversible plastic state due to energy dissipation; 2. the thermal damage of granite exhibits a porous structure at 50 ~ 600 ℃, which turns into a mixed pore-fracture structure at 600 ~ 800 ℃; 3. the granite matrix followed fractal distribution patterns after thermal treatment, demonstrating a quadratic correlation with temperature; 4. based on the fractal evolution of granite after high temperature, the temperature permeability model is derived. The new model is compared to the two commonly used models. The new model showed smaller fitting relative errors, and produced a better representation of the causes of plastic thermal damage, offering a promising solution for similar problems. Highlights: The classical exponential model between temperature and permeability is based on thermos-elastic theory. But in this paper, the development of plastic irreversible thermal damage is explained by energy dissipation Fractal theory is used to describe the evolution of thermal damage quantitatively Based on the fractal expression of thermal damage, a new temperature-dependent permeability model is deduced The fractal expression of thermal damage and the new temperature-dependent permeability model are verified by meso-CT test and gas permeability test [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented in which the editors discuss the 48th US Rock Mechanics/Geomechanics Symposium held at the University of Minnesota from June 1-4, 2014.

Depending on the type of failure mechanism, the strength of intact rock may be susceptible to the effects of scale. The influence of scale is apparent in the spalling of rock around underground openings. To reconcile field-scale observations, an understanding of the unconfined rock strength has to be appreciated from the standpoint of energy instability, particularly for instances in which the mechanism of failure involves propagation of fractures. Fracture energy principles were used to create a link between laboratory-measured uniaxial compressive strength (UCS) and rock failure as observed in spalling around circular openings. An Instability Line (IL) is proposed to predict the stresses above which spalling could occur with different opening diameters. Good comparisons were obtained for a range of dimensions from small boreholes to large tunnels. Based on field observations and analytical study, the results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the IL. The findings suggest that the effects of scale are limited by the crack initiation stress. For large-diameter openings, instability occurs immediately at crack initiation stress, whereas for smaller-diameter openings instability occurs at higher stresses post-crack initiation stress. Highlights: This paper aims to explain the discrepancies in the rock mechanics literature to predict spalling for small-diameter boreholes as against large-diameter tunnels. The failure mechanism of spalling is studied based on the framework of energy stability. An Instability Line is proposed based on the analytical study. The results of the analytical study compared well with published laboratory and field observations. The results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the Instability Line. The findings suggest that the effects of scale are limited by the crack initiation stress. [ABSTRACT FROM AUTHOR]

The paper applies the observation cycle to rock fracturing processes. Specifically, it repeats the observation cycle by going from the large (field) scale to increasingly detailed laboratory scales. With this, it is possible to determine that what appear to be tensile or shear fractures on the large scale are produced through a combination of tensile and shear mechanisms on the smaller scale. This involves observations with a variety of techniques that are briefly described. What is also emphasized is that observation has to go hand in hand with abstraction, i.e., modeling to help understand the observed phenomena. The paper, therefore, makes two contributions—a better understanding of rock fracturing processes and showing the importance of the observation cycle in a scientific process. [ABSTRACT FROM AUTHOR]

In this article the author responds to the comments on their article "Characteristic Curves for Deep Circular Tunnels in Poroplastic Rock," which appeared in the Volume 43, Number 2 issue in 2010. He addresses issues including pore pressure and effective stress jump at the elastic-plastic interface, unnecessary parameters and equations, and inapplicability of the long-term solution. He also clarifies the intent and scope of their paper and asserts that the paper complements current literature.

Numerical simulation of radon migration in fractured rock masses is crucial for environmental radon pollution protection, radon tracing technology and the co-exploitation of coal and uranium resources. This paper proposed a novel model based on a fractal discrete fracture network (DFN) to simulate radon migration in fractured rock masses. The paper provided a detailed method for determining the locations of three-dimensional fractures using the multiplicative cascade process. The governing equations for radon migration in fractures and the rock matrix were established and numerically solved using the finite element method. The study developed open-source software DFNRn (Discrete Fracture Network model for Radon migration), which was publicly available on GitHub. The software simulated radon migration in a fractured rock mass with a domain size of 100 m. The results demonstrate that fluid convection in fractures drives radon migration, which is dominant compared to the diffusion process. The accuracy of our proposed model was verified by comparing it with the measured data of the fractured rock. Furthermore, the study investigated the relationship between the percolation threshold of the fracture density P 32 ′ and the key parameters of the DFN. The results show that P 32 ′ decreased with the fractal dimension Df (1.4 ≤ Df ≤ 1.9) and normalization constant α (1.4 ≤ α ≤ 1.8) and is not correlated with the length exponent a (1.2 ≤ a ≤ 2.8). P 32 ′ decreased with the mean orientation angle and has no correlation with the Fisher constant. The radon source term of the rock matrix has the greatest impact on radon migration in fractured rock masses compared to the radon diffusion coefficient and permeability. Highlights: A new approach is proposed to model radon migration in fractured rock masses. An open-source software DFNRn was developed and is publicly available on GitHub. The procedure of the multiplicative cascade method in 3D space is given in detail. [ABSTRACT FROM AUTHOR]

The no-pillar longwall mining method (NPLM) by roof cutting technology (RCT) is an innovative longwall mining method proposed in China. Its essential technology is the employment of RCT to replace the chain pillar to safeguard the roadway. In this paper, a novel physical model test method is designed to investigate the protective mechanism of RCT for roadways in greater detail. Furthermore, the physical model test of RCT is conducted in consideration of the effect of broken roof rock bulking. Combined with numerical simulation, the results indicate that RCT can increase the degree of gangue bulking in goaf, hence providing effective support to the overlying strata and maintaining their integrity. Moreover, RCT can significantly reduce the abutment stress of the roadway roof and solid coal side, thereby ensuring the stability of roadway surrounding rock. This paper proposes three protection mechanisms of RCT for roadway based on the findings of the conducted research: I. Reducing the dimensions of roadway roof; II. Utilizing bulking gangue to support the overlying strata; III. Eliminating the cavities in the goaf adjacent to the roadway. The in situ application provides evidence that RCT is effective in controlling roadway deformation. This paper can serve as a reference for no-pillar mining and analogous geotechnical engineering physical model tests. Highlights: A novel two-dimensional physical model test method is developed that can achieve bulking after failure. Three protection mechanisms of roof cutting technology for the gob-side roadway are elucidated. The effect of roof cutting technology on the bulking of broken roof rock is analyzed by experiment. In conjunction with the in-situ application, the key to no-pillar longwall mining by roof cutting technology is proposed. [ABSTRACT FROM AUTHOR]

The phenomena of freezing damage in the cold regional tunnels have happened frequently. Therefore, in order to precisely calculate frost heaving force ( σ 1 ), based on the displacement method, a new analytical solution was proposed. The analytical solution presented in this paper was verified by the results of the model test and field measurement of σ 1 , the imperfect contact effect between frozen and unfrozen zones of surrounding rock was considered and the influence of the interaction between any two factors on frost heaving force was studied by means of orthogonal experiment and analysis of variance. The results show that: (1) the inner radius of lining ( r a ), excavation radius ( r 1 ) and freezing radius ( r f ) are highly significant for σ 1 and the interaction between the inner and outer radiuses of lining (i.e., changes of the size in inner and outer radius of lining) is also highly significant for frost heaving force ( σ 1 ); (2) the analytical solution presented in this paper can avoid the frost heaving force ( σ 1 ) with tensional property under the condition of the circular tunnel under hydrostatic pressure; and (3) sensitivity function shows that the larger Young's modulus of lining ( E I ), inner radius of lining ( r a ), freezing radius ( r f ), linear frost heaving rate ( β 0 ) and Young's modulus of frozen surrounding rock ( E II ) are, the more sensitive the factors E I , r a , r f , β 0 and E II are to σ 1 . It is expected that the results of this paper can give some novel insights for the anti-freezing design of tunnels. Highlights: Inner radius of lining is the more sensitive to frost heaving force. The interaction between the inner and outer radiuses of lining is highly significant. The ra, r1 and rf are highly significant for frost heaving force. The analytical solution can avoid frost heaving force ( σ 1 ) with tensional property. [ABSTRACT FROM AUTHOR]